In recent years, accompanying the expansion of the transmission capacity of optical communication, optical devices in which optical polarization division multiplexing is implemented are sometimes used. The optical polarization division multiplexing represents a transmission system in which two independent sets of data are transmitted at once using polarization multiplexed light that is formed by synthesizing signal lights whose directions of polarization are mutually orthogonal.
In an optical device in which optical polarization division multiplexing is implemented, the input light that is input to the optical device gets branched into two branched lights by an optical coupler, and the direction of polarization of one branched light is changed using a polarization converter. Then, the branched light having the changed direction of polarization and the branched light having the unchanged direction of polarization are synthesized using a polarization synthesizer, and the result is output as the polarization multiplexed light. From the perspective of achieving downsizing of the optical device; the optical coupler, the polarization converter, and the polarization synthesizer are mounted on, for example, a single substrate. As the optical coupler and the polarization synthesizer, for example, directional couplers are used that perform optical coupling of two waveguides.
[Patent Literature 1] Japanese Laid-open Patent Publication No. 2006-65089
[Patent Literature 2] Japanese Laid-open Patent Publication No. 2-287408
[Patent Literature 3] Japanese Laid-open Patent Publication No. 2011-34057
Regarding one of the two branched lights included in the polarization multiplexed light, a polarization synthesizer has the wavelength characteristic that the optical power goes on decreasing as the wavelength of the light approaches the end portion of a predetermined wavelength band assigned to the polarization multiplexed light (hereinafter, called an “operating wavelength band”). Thus, in an optical device in which optical polarization division multiplexing is implemented, there is a possibility of deterioration in the polarization multiplexed light due to such a wavelength characteristic of the polarization synthesizer. That is, regarding one of the two branched lights included in the polarization multiplexed light output from the polarization synthesizer, there is a risk that the minimum value of the optical power of the concerned branched light drops to a range that is equal to or below the specifications value allowed in advance at the end portion of the operating wavelength band.